Glycuronic Acid and Other Sugars
Glucuronic Acid
Glucuronic acid is essential for the conjugation/solubilization of
bilirubin
(from hemoglobin)
synthesis
– from NAD
+
-mediated oxidation of UDP-glucose (from glycogen pathway)
degradation
– conversion to xylulose, continue through pentose pathway
Fructose
Six carbon, forms
five membered ring
(non-reducing end is keto not aldehyde
fructose-6P is an intermediate of glycolysis –
hexokinase
can produce it from fructose
however
, most fructose absorbed by cells is converted into fructose-1P by
fructokinase
fructose-1P aldolase cleaves this into DHAP and glyceraldehyde (
Þ
glyceraldehyde-3P)
glyceraldehyde-3P and DHAP can be used for glycolysis or gluconeogenesis
Diseases of Fructose Metabolism:
(1) benign fructosurea
– no fructokinase, so fructose must be convertd to fructose-6P
hexokinase is less abundant and less active than fructokinase
fructose levels fall very slowly, and fructose frequently appears in the urine
(2) herediary fructose intolerance
– no fructose-1P aldolase, so fructose-1P builds up to toxic levels
interferes with metabolism in liver and kidney – potentially fatal (nausea, abdominal pain, hypoglycemia)
Galactose
Similar to glucose, but not as stable in ring form – phosphorylated by
galactose kinase
to Gal-1-P
UMP-galactose transferase
– galactose-1P + UDP-glucose
Þ
glucose-1P + UDP-galactose
UDP-galactose can be epimerized to UDP-glucose
Þ
glycogen/glycolysis/etc.
Diseases of Galactose Meabolism:
(1) galactose kinase deficiency
– buildup of galactose, produces galactosurea and cataracts
(2) galactosemia
– lack the transferase – toxic accumulations of galactose-1P, esp. in brain
produces severe mental retardation, cataracts, vomiting, hypoglycemia
Aldose Reductase
– uses NADPH to reduce glucose
Þ
sorbitol and galactose
Þ
dulcitol
sorbitol can be converted to fructose +NADH by sorbitol dehydrogenase, but there is no such reaction for dulcitol
can build up in eye to cause cataracts
Mannose
Mannose
Þ
mannose-6P
Þ
fructose-6P
Common Disaccharides
sucrose = glucose
a
1,2 fructose
maltose = glucose
a
1,4 glucose
lactose = galactose
b
1,4 glucose
